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Sai (サイ) is a ninja who joins Team 7 in Part II (Naruto Shippuden). Having been a member of The Foundation since he was a child, Sai has been trained to not possess or desire any type of emotion or camaraderie.

When first introduced, Sai was distinguished by his total lack of emotion which he often masks with a false, cheerful, yet transparent façade. After much mental conditioning and training while in Root, Sai had suppressed almost all of his emotions, laying all of his loyalty to whoever commands him, and thus making it easier for him to accomplish dangerous, unpleasant, or unethical missions. Due to this training and his lack of social experience and personal initiative, Sai has great trouble in understanding how bonds between people can motivate others. He often showcases this confusion after thinking about why Naruto would go so far to save Sasuke, even though Naruto had not been ordered to do so.

After spending more time with Team Kakashi, Sai comes to appreciate bonds, particularly the bond that exists between Naruto Uzumaki and Sasuke Uchiha. This causes him to break out of his emotionless shell and he starts to remember the times he had with his own "brother", Shin. In doing so, Sai was able to start giving genuine smiles. Wanting to experience personal bonds for himself, Sai sets out to make as many friends as he can. However, since he is learning most of this out of books and by trial and error, he often accidentally ends up offending people with his nicknames and conversation. He has come to develop a more genuine, pleasant and thoughtful personality, while also showing signs of more emotion, such as enthusiasm and occasionally nervousness.